Compile zend_execute.c with special CFLAGS. For GCC, INLINE_CFLAGS
contains -O0 to disable optimizations. This can be disabled by using
the appropiate parameter.

Don't set DEBUG_CFLAGS to -g, if -g is already in CFLAGS

Explicitly check for C++ preprocessor, otherwise autoconf forces it onto
us at the wrong place (subsequent autoconf checks failed).

libzend -> Zend